1. Field of the Invention
The present invention generally relates to induction heating and, more particularly, to a formable composite magnetic flux concentrator for use in induction heating applications. The present invention also relates to a method of making the concentrator.
2. Description of the Prior Art
Induction heating is a relatively efficient manner of generating heat in an electrically conductive part. When changing electrical current flows in an induction heating coil, it will cause a changing magnetic field to be generated about the coil. If the electrically conductive part is placed within the coil, then the changing magnetic field will induce a current to flow around the part which will generate heating of the part due to its inherent electrical resistance to the current flow. No contact is necessary between the coil and part. The magnetic flux field is passed through an air gap between the coil and part.
By placing a composite magnetic flux concentrator on the induction heating coil, a stronger magnetic field is generated in the air gap between the coil and part. The stronger the magnetic field, the faster and more efficiently the part will be heated. The magnetic flux concentrator is formed of a magnetically conductive material that, when placed on the coil, creates a more efficient and controlled magnetic flux path and increases the intensity of the magnetic flux field.
The use of a magnetic flux concentrator also has the following additional benefits. The concentrator (1) increases the magnetic coupling into the part, thus using less energy; (2) decreases the potential hazardous magnetic and RF exposure to which machine operators are exposed; (3) defines the specific area that is to be induction heated, thereby holding the heat affected zone to a controlled or minimum which is metallurgically beneficial to the part; and (4) allows the focusing/shielding of the magnetic energy into/from zones that would not otherwise be achievable without the use of the concentrator.
There are basically three different types of prior art magnetic flux concentrators in commercial use. The first type of prior art concentrator is provided in the form of laminations of numerous thin sheets of steel. Each sheet is electrically insulated from the other sheets. The laminations are custom fitted to the shape required and placed side by side over the coil. However, undesirably high eddy currents are generated within the sheets and excess heat energy is produced within the concentrator. At higher frequencies, thinner laminations must be used in order to keep eddy current generation to a minimum. Because of physical thickness limitations, this first type of concentrator is limited to relatively low frequency applications. Also, excess heat production requires cooling of the laminations which is labor intensive and expensive. Thus, the problems associated with the laminated type of concentrator is the amount of labor required for custom fabrication, the expense and difficulty in cooling, the difficulty in repairing laminations, and the limitation of use to relatively low frequencies.
The second type of prior art concentrator is a ferrite. The ferrite is an iron alloy crystal that is pressed into a form that has in itself been custom fitted to the coil. The formed substance is then fired at very high temperature in an oxygen-free oven to form a ceramic-like material. Being of a ceramic-like material, the concentrator will fracture if heating is not uniform. When a part is heated it increases in heat energy and, in turn, radiates heat energy into the work coil and the concentrator. The radiant heating oftentimes causes uneven heating of the material. Being a hard, stone-like material, the ceramic-like concentrator is all but impossible to water cool, without generating thermal stresses.
The third type of prior art concentrator is a machinable bar made by combining very small insulated iron powdered metal particles and small amounts of binder. This combination is then placed in a mold and pressed with a force of over 2000 pounds per square inch while heat is applied. Once formed the bar must be machined to fit the coil shape needed. This type of concentrator is able to work at higher frequencies than the laminated material because of the insulating abilities and low hysteresis losses of the small powders. However, when large time variable magnetic fluxes are applied for long periods of time, the need to water cool the concentrator still exists. The bar concentrator is expensive to form, labor intensive to machine, and difficult to water cool.
Consequently, a need still exists for improvement of magnetic flux concentrators and of techniques for fabrication which will overcome the problems associated with the prior art types of concentrators described above.